Hi,

There are some dyno tests out there but they compare mid-performance cars that are only slightly tuned or not tuned at all. Such tests are not relevant imo since most of us are interested in the effect of different cats or going catless on the max. power output of highly tuned performance engines.

The mistake almost everybody makes is to linearly extrapolate the results of said tests to other cars. This is completely wrong for the following reason:

The power required for gases to pass an obstacle (in this case the exhaust gases passing through the cat) is proportional to the 3rd power of the gas velocity - and the exhaust gas velocity is roughly proportional to the rpms and the displacement!! (And this required power is, of course, the power lost at the crankshaft). So for highly tuned, high-revving cars with a fairly high displacement, the results are completely different than for the mentioned tests. In the latter, the exhaust is almost not limiting but at high revs it becomes more and more relevant really really quickly (with the 3rd power of the rpms!!).

I'm not a fan of going catless on a street car because it smells, is obnoxiously loud and pollutes the air but I would like to know what's up. So does anybody have relevant data to share? (Please no comparison of e.g. 3in catless vs. 2.25in stock cat; only one parameter changed at a time, everything else is meaningless)